Process for the deposition of thick chromium films from trivalent chromium plating solutions and article so produced

ABSTRACT

A process for plating thick chromium coatings for engineering applications comprises depositing a thin initial layer from a low concentration chromium III/thiocyanate bath and depositing the bulk of the remaining thickness from a relatively higher concentration chromium III/thiocyanate bath. Deposits produced by this two-stage process are more cohesive and smoother than those obtainable by plating the entire thickness from the high concentration bath alone.

TECHNICAL FIELD

This invention relates to the deposition of thick films from chromiumelectroplating solutions in which the source of chromium comprises anaqueous solution of a chromium (III)--thiocyanate complex. In thiscontext, the term "thick film" means a film of greater than five micronsthickness.

BACKGROUND ART

Conventionally, chromium has been plated from aqueous chromic acid bathsprepared from chromic oxide (CrO₃) and sulphuric acid. Such baths, inwhich the chromium is in hexavalent form, are characterized by lowcurrent efficiency. The chromic acid fumes emitted as a result ofhydrogen evolution also present a considerable health hazard.Furthermore, the concentration of chromium in such baths is extremelyhigh, leading to problems of waste or recovery because of so-called"drag-out" of chromium compounds into the rinse tanks which follow theplating bath.

To overcome many of the disadvantages of hexavalent chromium plating, ithas been proposed to plate chromium in trivalent form. One such processfor plating chromium from an aqueous solution of a chromium(III)--thiocyanate complex is described in UK Pat. No. 1,431,639 and itsequivalent U.S. Pat. No. 4,062,737. Another such process is described inout UK patent application No. 24734/77 and its equivalent U.S. Pat. No.4,161,432 which describes a chromium plating solution and process inwhich an aqueous solution of a chromium (III)--thiocyanate complex isagain employed but in which a buffer material supplies one of theligands to the chromium complex. The buffer material is selected fromamino acids (e.g., glycine, aspartic acid), peptides, formates, acetatesand hypophosphites.

These trivalent chromium plating processes do not give off chromic acidfumes. They are of high efficiency with a wide plating range and goodcovering power. A very much lower amount of chromium is needed in thebath than is the case with hexavalent processes, thus reducing theproblems associated with "drag-out". Concentrations of chromium haveranged from 0.30 to 0.5 Molar.

Although the trivalent chromium plating processes of UK Pat. 1,431,639and UK patent application No. 24734/77 overcome all the majordisadvantages of hexavalent plating, the appearance of the depositedchromium is generally somewhat darker. While this color is quiteacceptable or even preferable for many applications, it is advantageousto be able to plate lighter colored chromium with a trivalent process.In UK patent application 44177/78, chromium (III)--thiocyanate baths aredisclosed whose chromium concentration is far below the generallyaccepted level for efficiency and bath stability. Such baths give asignificantly lighter colored deposit. The concentration of chromium inthese baths is less than 0.03 M and preferably less than 0.015 M.

DISCLOSURE OF THE INVENTION

Chromium plating, besides its decorative applications, is also used forengineering purposes. Because of its hardness, low friction andcorrosion resistance, it is used to provide, for example, a wearresistant coating on the surface of a sliding machine part or to providesuch a coating on screws or bolts. For such applications, it isgenerally necessary that the thickness of the plated chromium be verymuch greater than in decorative applications. Typically, decorativechromium is less than one micron in thickness whereas "engineering"chromium needs to be of the order of tens of microns thickness. Suchthicknesses have hitherto been achievable only with hexavalent chromiumplating. Attempts to plate thick chromium (above 5 microns) fromtrivalent baths such as those of UK Pat. No. 1,431,639 and UK patentapplication No. 24734/77 have resulted in coarse, matt deposits withpoor cohesion.

The present invention provides a process of electroplating an articlewith a layer of chromium exceeding five microns in thickness comprisingthe step of electroplating the article with an initial relatively thinlayer of chromium from an equilibrated aqueous solution of a chromium(III)--thiocyanate complex of relatively low chromium concentration, andplating the major proportion of the remaining thickness in one or moresteps from an equilibrated aqueous solution of a chromium(III)--thiocyanate complex of relatively higher concentration.

The invention is based on the discovery that if an initial thin layer isdeposited from a dilute Cr (III)--thiocyanate bath, subsequent thickdeposits from a more concentrated, higher rate Cr (III)--thiocyanatebath have much better properties of cohesion and surface smoothness.Preferably, the low concentration is less than 0.03 Molar. In thispreferred process, only the initial thin layer is deposited from thelower concentration solution, the entire remaining thickness beingplated from more concentrated solution.

Alternatively, alternate thick and thin layers may be plated from thehigher and lower concentration baths respectively.

The preferred thickness of the initial thin layer is less than 1000Angstroms.

The preferred chromium plating solutions include an amino acid as abuffer material, providing at least one of the ligands for the complex.

The preferred current densities are in the range 40-50 mA cm⁻² for theinital layer plating step and 50-120 mA cm⁻² for the concentratedsolution plating step.

DETAILED DESCRIPTION OF THE INVENTION

In plating thick chromium according to the invention, chromium has beenplated onto standard steel test panels in thicknesses ranging from 10-75microns. The steel test panels were, in some cases, first plated withbright nickel to a thickness of 10-12 microns.

Chromium seeding layers were plated from a bath having 0.003 M chromiumconcentration to a thickness of not more than 1000 Angstroms. Further,chromium was deposited from a bath having 0.1 M chromium concentration.In some cases, deposition of the seeding layer was followed by a singleplating step from the 0.1 M bath to deposit the remainder of the film.In other cases, a few microns from the 0.1 M bath were alternated with aflash layer from the 0.003 M bath.

A comparative example was plated from the 0.1 M bath alone and found tohave a surface profile center line average (CLA) of 75 microinches. TheCLA's of the samples plated according to the invention were much lower,down to seven microinches.

ESCA measurements of the deposit indicate that the low concentrationchromium is very pure whereas the high concentration chromium depositincludes chemically bound oxygen and sulphur. It is believed that, sincethe initial thin layer is very pure and uniform, it acts as a seedinglayer for the remainder of the deposit which limits its granularity. Theoverall thick film is thus more cohesive and less friable than films ofthe same thickness deposited from the higher concentration bath alone.The light color of the deposited chromium from low concentration baths,as described in UK patent application No. 44177/78, may also be relatedto purity. Conversely, the entire range of low chromium concentrationbaths disclosed in that application is expected to be beneficial indepositing thick films of chromium. This range was 0.0002-0.025 M, witha postulated upper limit of 0.03 M.

The invention will now be described further with reference to thefollowing examples:

COMPARATIVE EXAMPLE I

A relatively high concentration of trivalent chromium plating solutionwas prepared in the following manner:

(a) 60 grams of boric acid (H₃ BO₃) were added to 750 ml of deionizedwater which was then heated and stirred to dissolve the boric acid.

(b) 33.12 grams of chromium sulphate (Cr₂ (SO₄)₃.15H₂ O) and 32.43 gramsof sodium thiocyanate (NaNCS) were added to the solution, which was thenheated and stirred at approximately 70° C. for about 30 minutes.

(c) 13.3 grams of DL aspartic acid (NH₂ CH₂ CH(COOH)₂) were added to thesolution which was then heated and stirred at approximately 70° C. forabout 3 hours. During this time, the pH was adjusted from pH 1.5 to pH2.8 very slowly with 10% by weight sodium hydroxide solution. Once thepH of 2.8 was achieved, it was maintained at this value for the whole ofthe equilibration period.

(d) Sufficient sodium chloride was added to the solution to make itapproximately 1 M concentration and 0.l grams of FC 98 (a wetting agentproduced by 3 M Corporation) was also added. The solution was heated andstirred for an additional 30 minutes.

(e) The solution pH was again adjusted to pH 2.8 with a sodium hydroxidesolution.

(f) The solution was made up to one liter with deionized water which hadbeen adjusted to pH 3.0 with 10% by volume of hydrochloric acid.

The final solution is of the following composition:

0.1 M chrome sulphate--Cr₂ (SO₄)₃.15H₂ O;

0.4 M sodium thiocyanate--NaNCS;

0.1 M aspartic acid--NH₂ CH₂ CH(COOH)₂ ;

60 g/l boric acid--H₃ BO₃ ;

60 g/l sodium chloride--NaCl; and

0.1 g/l FC 98 wetting agent.

This electroplating solution was introduced into a plating cell. Aplatinised titanium anode and a steel sample panel was cathode wereimmersed in the cell. The steel panel had an overcoating of 10-12microns of bright nickel. A plating current of 75 mA cm⁻² was passedbetween the electrodes for 90 minutes. A layer of chromium of 20.9microns thickness was deposited.

EXAMPLE I

A second lower concentration chromium plating solution was made up asfollows. A solution was prepared in exactly the same manner as describedin Comparative Example I except that one-half the quantity of sodiumthiocyanate was used, resulting in a sodium thiocyanate concentration of0.2 M. 30 mls of this solution were made up to one liter with a solutioncontaining 60 grams per liter of boric acid and 60 grams per liter ofsodium chloride.

The final lower concentration solution had essentially the followingcomposition;

0.003 M chrome sulphate;

0.006 M sodium thiocyanate;

0.003M aspartic acid; 60 g/l boric acid; and

60 g/l sodium chloride.

The lower concentration electroplating solution was introduced into aplating cell having a platinized titanium anode and a steel sample panelas cathode. A plating current of 40 mA cm⁻² was passed through the cellfor 240 seconds to deposit an initial layer of chromium estimated to benot more than 1000 Angstroms in thickness.

The panel was then transferred without rinsing to a second plating cellcontaining a higher concentration chromium electroplating solution ofthe same composition as that of Comparative Example I. A plating currentof 75 mA cm⁻² was passed through the cell for 180 minutes to deposit amuch thicker layer of chromium on top of the initial thin layer. Thefinal thickness of the chromium layer was 21.6 microns.

This thick layer appeared smooth and reflective to the eye. The CLA ofthe surface was seven microinches (0.178 microns). The deposit was lessfriable and more cohesive than that of Example I.

EXAMPLE II

The process of Example I was repeated in a series of experiments usingthe same two plating solutions, although in some cases the wetting agentwas omitted. This appeared to improve the characteristics of the depositeven further by reducing granularity. Films ranging from 10 to 75microns thickness were plated. Current densities for plating from thelow concentration bath were in the range 40-50 mA cm⁻². Currentdensities for plating from the high concentration bath were in the range50-120 MA cm⁻².

CLA measurements on some of these samples lay in the range 7-11.2microinches.

EXAMPLE III

Using the same solutions as for Example I, and starting with the lowerconcentration solution, alternate layers of chromium were deposited on asteel sample panel from the two solutions.

The steel panel was first connected as cathode in the low concentrationbath and a current density 40 MA cm⁻² was passed for 240 seconds toproduce a thin initial layer of chromium of no more than 1000 Angstromsthickness. The panel was transferred, without rinsing, to the highconcentration bath and plated at a current density of 50 MA cm⁻² for 30minutes to produce a thicker layer of chromium. The panel was thentransferred back to the low concentration bath and plated for 2 minutesat 40 mA cm⁻². The alternate plating for 30 minutes in the highconcentration bath and 2 minutes in the low concentration bath wascontinued for a total time of 215 minutes.

In all a thickness of 16.9 microns of chromium was deposited. The finaldeposit was cohesive, smooth and nonfriable and had a CLA of 8microinches (0.2 microns).

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:
 1. A process of electroplating an articlewith a layer of chromium exceeding five microns in thickness comprisingthe step of electroplating the article with an initial relatively thinlayer of chromium from a first equilibrated aqueous solution of chromium(III)--thiocyanate complexes of relatively low chromium concentration ofabout 0.003 M, and plating the major proportion of the remainingthickness in one or more steps from a second equilibrated aqueoussolution of chromium (III)--thiocyanate complexes of relatively higherconcentration of about 0.1 M.
 2. A process of claim 1 wherein the entireremaining thickness after plating of the initial thin layer is platedfrom the more concentrated solution.
 3. A process of claim 1 wherein theremaining thickness of the chromium layer is plated alternately from thehigh and low concentration solutions in thick and thin layersrespectively.
 4. A process of claim 1 wherein the initial relativelythin layer plating is terminated after deposition of a layer whosethickness is less than 1000 Angstroms.
 5. A process of claim 1 whereinthe electroplating solutions each include an amino acid as a buffermaterial providing at least one of the ligands for the complex.
 6. Aprocess of claim 1 wherein the solutions include a wetting agent.
 7. Aprocess of claim 1 wherein the current density while the initial thinlayer is being plated is in the range 40-50 mA cm⁻².
 8. A process ofclaim 1 wherein the current density during plating from the moreconcentrated solution is in the range 50-120 mA cm⁻².
 9. An articleplated by a process as claimed in claim
 1. 10. A process ofelectroplating an article with a layer of chromium exceeding fivemicrons in thickness comprising the step of electroplating the articlewith an initial relatively thin layer of chromium from an equilibratedaqueous solution of chromium (III)--thiocyanate complexes of arelatively low chromium concentration less than 0.03 M, and plating themajor proportion of the remaining thickness in one or more steps from anequilibrated aqueous solution of chromium (III)--thiocyanate complexesof a relatively high chromium concentration greater than 0.03 M;therelatively low concentration solution comprising: 0.003 M chromesulphate; 0.006 M sodium thiocyanate; 0.003 M aspartic acid; 60 g/lboric acid; and 60 g/l sodium chloride; andthe relatively highconcentration solution comprising: 0.1 M chrome sulphate; 0.4 M sodiumthiocyanate; 0.1 M aspartic acid; 60 g/l boric acid; and 60 g/l sodiumchloride.